Explanation is an important issue in building computer-based interactive design environments in which a human designer and a knowledge system may cooperatively solve a design problem. We consider the two related problems of explaining the sys­tem’s reasoning and the design generated by the system. In particular, we analyze the content of explanations of design reasoning and design solutions in the domain of phys­ical devices. We describe two complementary languages: task-method-knowledge mod­els for explaining design reasoning, and structure-behavior-function models for explain­ing device designs. INTERACTIVE KRITIK is a computer program that uses these repres­entations to visually illustrate the system’s reasoning and the result of a design episode. The explanation of design reasoning in INTERACTIVE KRITIK is in the context of the evolving design solution, and, similarly, the explanation of the design solution is in the context of the design reasoning.